The broad, long-term objective of the research is to arrive at a comprehension of the structure of mammalian ribosomes in sufficient detail so as to be able to provide a coherent account of their function in protein synthesis. The research proposed here is aimed at an aspect of this project - the nature of the association of the ribosomal proteins with the rRNAs. In these studies we make use of two the chemistry of RNA-protein interactions relevant to ribosome structure. alpha-Sarcin is a RNase that cleaves the phosphodiester bond on the 3' side of the guanosine at position 4325 in a universal, 12 nucleotide, purine-rich sequence in 28S rRNA - the longest conserved sequenced of bases in all of rRNA. Ricin is a RNA N-glycosidase that catalyzes the depurination of the 5'adjacent A4324. There are nearly 7,000 nucleotides in mammalian ribosomes yet the toxins catalyze only a single covalent modification which, in each instances, inactivates the ribosome and accounts entirely for the toxicity. This approach, i.e. the use of toxins, has value since an analysis of the mechanism of action of these correlates of structure. The discovery of functional correlates of structure is a prime purpose of the in E. coli 23S rRNA and to analyze the effects of the mutations in vivo in the a-sarcin/rich (S/R) domain the partial reactions of protein synthesis. (2) To analyze the identity elements in the S/R domain RNA for recognition by alpha-sarcin and by ricin-A-chain (RA). (3) To study the binding of a-sarcin, RA, and the elongation factors (EF- 1/EF-2 and EF-Tu/EF0G) to an oligoribonucleotide that reproduces the S/R domain in 23S/28S rRNA. (4) To determine the nature of a ribosomal mutation that confers on CHO cells ricin resistance. (5) To construct systematic amino acid deletions in the ribonuclease bardase and to analyze the effects on catalysis and on binding to the protein inhibitor barstar. (6) To participate with Dr. Peter B. Moore in a collaboration aimed at establishing the structure of a S/R domain olligoribonucleotide by NMR spectroscopy.